Rmpi and C Code, where to get the communicator

Hello,

I try to write parts of my code in C to accelerate the for-loops. But 
basic operations I want to do in R (e.g. start cluster). My R code looks 
something like this:

library(Rmpi)
mpi.spawn.Rslaves()
mpi.remote.exec(....)
dyn.load("test.so")
erg <- .Call("test", ....)
....
mpi.close.Rslaves()
mpi.quit()

And my C function looks something like this:

#include <mpi.h>
#include <R.h>
#include <Rdefines.h>
#include <Rinternals.h>
SEXP test (SEXP a, ..){
   int rank;
   comm ??????
   MPI_Comm_rank (comm, &rank);
   ......
}

For all MPI functions I need the right communicator (MPI_Comm *comm;) to 
communicate with the existing slaves. This variable will be generated by 
spawning the Rslaves (mpi.spawn.Rslaves). But how to get to this 
variable in my C function?
Is the communicator variable somewhere stored in a R variable?

( I use R 2.6.2 and Rmpi 0.5-5)

Thanks
Markus